1. Field of the Invention
This invention relates to a mobile test apparatus and a method of making precise thermal performance measurements of process piping and systems, and more specifically to a testing method and apparatus for evaluating thermal insulation systems of pipelines of various media at various temperatures.
2. Prior Art
U.S. Pat. No. 4,978,229 is directed to a method and apparatus for testing thermal conductivity of insulation placed over a test apparatus. While this device is an improvement over the prior art in recognizing that the shape of the insulation contributes to the conductivity of the insulation, this device is not designed, or even capable, of measuring the conductivity of a piping system which is transporting fluid during the test.
U.S. Pat. No. 4,396,300 is directed to a test apparatus for testing the heat transfer and friction characteristics of a tube. It appears that the disclosed apparatus is designed to test an uninsulated tube which would be evaluated for possible use as a heat exchanger tube. There is no provision for eliminating end effects discussed in this reference since the piping segment is a continuous loop. Furthermore, there is no provision for a static boil off test for cryogenic fluids, only a flow through test.
Precise measurement of thermal performance of piping systems is advantageous in certain applications. As technology advances, the use of cryogenics will be more and more commonplace. As an example, hydrogen may provide a more common fuel source. Liquification of hydrogen would then be appropriate for storage in many applications. When handling liquefied gas, i.e., which is typically done at very cold temperatures, there is often a need for cryogenic piping systems.
In the past, data has been obtained from tests of specific segments of a system such as a segment of insulation for a pipe as described in U.S. Pat. No. 4,978,229. The data was then extrapolated as segments were conceptually combined together to approximate the thermal performance of piping systems. Tables provided by certain manufacturers including Chart-CVI of Columbus, Ohio, Chart-MVE, and PHPK include an estimated heat leak rate of some components utilized in the cryogenic piping systems. However, as components are combined together, different components are utilized together, and an approximation, or extrapolation, has been employed to provide an expected thermal performance criteria for the system. However, in field installations, the actual performance values for the insulation system can be as much as 10 to 100 times worse than the ideal laboratory values. Accordingly, a need exists to more definitively predict the thermal performance of actual pipelines and piping systems.
Heat leak measurements for higher performance insulation systems are by nature difficult since small heat leak errors can be very large with respect to the desired measurement. Variables such as external ambient conditions like wind, temperature, humidity, solar radiation, etc. could contribute large errors. Accordingly, a need exists to evaluate a piping configuration with test equipment which can be easily reproduced and provide a standard.
Consequently, it is a primary object of the present invention to provide a method and apparatus for obtaining data to measure the heat leak rate of cryogenic pipelines.
It is a further object of the present invention to provide a method and apparatus for comparing the thermal performance of piping systems.
Another object of the present invention is to provide a method and apparatus which is easily employed to provide the thermal performance of actual full-scale pipelines and piping systems.
Accordingly, the present invention provides a test apparatus and method to measure the heat leak rate of piping systems. Cold boxes are connected to each end of the piping systems. The test apparatus may be employed in one of two ways: a boil-off method for pipelines carrying cryogenic fluid, or a flow-through evaluation. For the boil off method, the flow rate of boil off is determined for the system, and the heat leak rate may be calculated as the boil-off flow rate times the latent heat of vaporization. For the flow through method, heat leak rate is equal to the liquid mass flow rate times the specific heat and the change in temperature across the system.
The piping system is preferably shrouded with a heater shroud to practically eliminate any environmental effects. Bellows at either end of the piping system allow for thermal expansion and contraction while being equipped to eliminate heat transfer from the pipe ends. Low conductivity pipe supports minimize thermal contact to reduce possible sources of errors. A plurality of temperature and pressure measurement sensors monitor temperatures and pressures at a number of locations to determine the pressures and temperatures at a number of locations along a piping system. Flow meters are also utilized at various locations to evaluate mass flow rates through the piping system. These temperatures, pressures and mass flow rates are utilized to provide the thermal performance characteristics of the piping system.